There are all sorts of indicators you can look at, not just radiometric dating. For starters, if mountains were formed quickly, this would generate enormous stresses and release enormous amounts of heat. This would produce all sorts of minerals that simply don’t form under slow, gradual processes. Here are a couple of examples:
There are also a lot of types of rock (sedimentary rock in particular) that cannot form quickly. Anything formed from microscopic particles (e.g. shale) needs long periods of time in very still or slow-moving water in order for the particles to settle out. This is simply a consequence of Stokes’s Law.